1. Field of the Invention
The present invention relates to an engine control apparatus to judge a fault of the sensor.
2. Discussion of Background
A system in which an engine control apparatus capable of judging a fault of the sensor is used generally has the construction as shown in FIG. 4, wherein a reference numeral 1 designates an engine, a numeral 2 designates an engine revolution speed sensor which detects the revolution number of the engine, which is attached to the engine, and a numeral 3 designates an engine control apparatus. The engine revolution speed sensor 2 generates a signal a having a frequency in proportion to the revolution number of the engine 1 and a signal b for discriminating the cylinders of the engine, the signals a, b being supplied to the engine control apparatus 3. The engine control apparatus 3 is adapted to perform a fuel control and an ignition timing control to the engine on the basis of the signals of the revolution speed sensor 2 and to detect that the revolution speed sensor 2 has become faulty. When the engine control apparatus 3 makes the judgment of a faulty state of the sensor 2, it operates the entire system to the fail-safe direction.
FIG. 5 is a timing chart for illustrating a conventional method of judging a faulty state in a revolution speed sensor.
FIGS. 5a and 5b show a timing in each output signal a, b; FIG. 5c shows a timing to detect the cylinders of the engine 1 and the cylinders discriminated thereby; and FIG. 5d shows a timing to detect the fault of the revolution speed sensor 2. In the conventional method of judging the fault of the revolution sensor, the judgment of the fault of the revolution sensor 2 was made when the level of the output signal a or the output signal b of the revolution sensor 2 does not change for a predetermined time t.sub.1 or longer during the operation of the engine as shown in FIG. 5.
Although the conventional technique of judging the fault of the sensor in the engine control apparatus correctly judged the fault of the sensor when the output signal of the revolution speed sensor was not provided for a predetermined time or longer, the following disadvantage was found. Namely, it was impossible to make the judgment of the fault of the sensor when a state of no change in the output signal of the revolution speed sensor continued for a time t.sub.2 (t.sub.2 &lt;t.sub.1) due to contact failure in the wiring between the revolution speed sensor and the engine control apparatus or external noises on the wiring, or when an abnormal signal was outputted from the revolution speed sensor. Further, there was a problem of shortening the lifetime of the engine if an inadequate cylinder was selected by the abnormal signal and the fuel supply and ignition control were made on the inadequate cylinder.